The present invention relates to a projection device for displaying at least one of a two-dimensional and/or three-dimensional scene or of content. The present invention relates in particular to a holographic reconstruction in a projection way. In particular the present invention relates to a projection display device using an optimized illumination for increased peak brightness and dynamic range. Such display devices are required mostly for projection applications like in cinemas, vehicle applications or similar applications. However, other applications are also possible.
Furthermore, the present invention relates also to a method for displaying at least one of a two-dimensional and/or three-dimensional scene or of content, in particular for generating a holographic reconstruction.
The present projection device is adapted for displaying two-dimensional (2D) and/or three-dimensional (3D) images. It shall be understood that two-dimensional images or three-dimensional images also include two-dimensional or three-dimensional contents or movies.
The field of application of the present invention includes preferably projection display devices for the three-dimensional presentation of holographic images.
In a commercially available projection display device for the presentation of two-dimensional images or movies/videos it is necessary to realize an increased brightness, high image quality and high contrast. The information to be presented is written into a spatial light modulator device of the projection display device. The light which is emitted by an illumination device comprising at least one light source is modulated with the information that is written into the spatial light modulator device, where the light modulated with the information is then projected by a projection system to a screen or similar. To achieve a high quality of the preferably three-dimensional presentation of the information written into the spatial light modulator device, a defined collimation of the wave fronts that are coupled out of the illumination device is necessary in addition to a homogeneous illumination of the entire surface of the spatial light modulator device. This is of high importance for holographic presentations in the form of a reconstruction that is to be generated. The holographic information, which can for example be an object that is composed of object points of a three-dimensional scene, is normally encoded in the form of amplitude values and phase values in the pixels as modulation elements of the spatial light modulator device. The encoded object points are generated by the wave field that is emitted by the spatial light modulator device.
A complex value which serves to modulate both the phase and the amplitude of a wave front cannot be displayed satisfactorily directly in a single pixel of a conventional spatial light modulator device. The modulation of only one value per pixel, i.e. a phase-only or an amplitude-only modulation, however only results in an insufficient holographic reconstruction of a preferably moving three-dimensional scene. A direct and thus optimal—in the sense of generalized parameters—representation of the complex values can only be achieved by a complex valued modulation preferably at the same plane and at the same time in the spatial light modulator device. Depending on the actual type of spatial light modulator device, various methods are known to achieve a simultaneous modulation of both parts of the complex values to be displayed.
In document U.S. Pat. No. 7,551,341 B1 a serial modulation display is disclosed comprising two spatial light modulators in the form of two DLP (digital light processing) which are arranged in series. These two DLP are combined by an optical transfer system. FIG. 1 shows this serial modulation display in detail. This projection display 10 comprises a light source 12, two DLP 14 and 20 and a projection lens 28 to project an image to a screen 29. Transfer optics 26 is provided to transfer light from the first DLP 14 to the second DLP 20. The transfer optics 26 also includes a blur function. The DLP 14 and 20 each comprises a plurality of controllable elements 16 and 22. These elements can be switched between ON or OFF states by a control circuit 18. When the element 16 is in its ON state, the element 16 allows incident light that hits the element to pass to a corresponding area of the second DLP 20. When the element 16 is in its OFF state, the light that passes from the element 16 to the second DLP 20 is diminished. Thus, no light from the element 16 reaches the second DLP 20. Each element 22 of the second DLP 20 can be controlled to select light that is incident on the element 22 from the first DLP 14 that is transmitted to a viewing area. The pattern of light incident on the second DLP 20 is determined from the configuration of the first DLP 14 and the transfer function of the transfer optics 26. This means which elements 16 are ON and which elements 16 are OFF.
Generally, a projection system like the serial modulation display of U.S. Pat. No. 7,551,341 B1 can comprise a light source device, two spatial light modulators, e.g. two DLP, and an optical system, e.g. a lens, arranged between the first and the second spatial light modulator. FIG. 2 shows a simplified schematic projection system having a light source device LS, two spatial light modulators DLP 1 and DLP 2 designed as DLPs and an absorber A. Because the two spatial light modulators DLP 1 and DLP 2 comprise reflective elements the real beam path is therefore different.
This projection system shown in FIG. 2 comprises thus the spatial light modulator DLP 1 as first spatial light modulator and the spatial light modulator DLP 2 as second spatial light modulator and uses a lens L between the first spatial light modulator DLP 1 and the second spatial light modulator DLP 2. Each pixel of the first spatial light modulator DLP 1 is imaged to a dedicated pixel of the second spatial light modulator DLP 2. Variations can exist where e.g. the first spatial light modulator and the second spatial light modulator do not necessarily have to comprise an identical number of pixels.
By setting a pixel of the first spatial light modulator DLP 1 to a black state (OFF state) by tilting a mirror element M of the first spatial light modulator DLP 1 accordingly in the direction of the absorber A to absorb this light incident on the absorber A, the illumination intensity of the dedicated pixel of the second spatial light modulator DLP 2 is zero, i.e. no light is incident on the dedicated pixel. By a binary sequence of ON and OFF states of the mirror element M of the first spatial light modulator DLP 1 also a grey level of illumination for the pixel of the second spatial light modulator DLP 2 can be generated. By making use of this fact and in addition by setting a suitable grey level on the second spatial light modulator DLP 2 the contrast of the content as shown on the projection system can then be increased compared to a single spatial light modulator system. But it should be noted that the maximum image (pixel) brightness cannot be larger than but only smaller than or equal to the brightness of the light source device LS which can be designed as laser light source.
For relative calculations, the incoming light intensity from the light source device LQ on the first spatial light modulator DLP 1 is set to 100%. In the special case that the content to be shown on the projection system or projection display is a white screen, then it is assumed that in the projection system, where all pixel of the first spatial light modulator is set to “ON-state”, the illumination intensity of the second spatial light modulator DLP 2 is also still close to 100%.
For the sake of simplicity it is also assumed that the transmittance of the second spatial light modulator DLP 2 is 100% in case of a white screen.
There are however disadvantages relating to such a projection system.
One of it is the energy loss. For an image scene with mostly dark content, that means large relatively dark areas and only a few maximum brightness objects, the light source device has to deliver its maximum brightness. Accordingly, in the projection system shown in FIG. 2 a large part of the illumination light will be filtered out and get lost in between the first spatial light modulator and the second spatial light modulator. The illumination intensity of a single white pixel on the second spatial light modulator will never be larger than the maximum brightness of the expanded light source device which is given by the first spatial light modulator, independent of the content on the second spatial light modulator.
A second main disadvantage is given by the peak brightness. For any scene with at least one spot with maximum brightness, the maximum power of the light source device is required. For any scene, the brightness provided by the light source device can never be smaller than the maximum brightness in any portion of the scene. Also peak brightness of selected highlights in the scene can never be higher than maximum brightness of the expanded light source device given by the first spatial light modulator.